July 20, 1993
12:00 AM (EDT)

News Release Number: STScI-1993-18

Hubble Space Telescope Finds a Double Nucleus in the Andromeda Galaxy

The full news release story:

A team of astronomers using NASA's Hubble Space Telescope has
discovered a "double nucleus" in the center of the neighboring
spiral galaxy M31, located in the constellation Andromeda.

"Hubble shows that the M31 nucleus is much more complex than
previously thought," says Dr. Tod R. Lauer of the National
Optical Astronomy Observatories, Tuscon, Arizona.

A nucleus is a dense clustering of stars at the very center of a
galaxy.

The astronomers report that the brighter member of the double
nucleus might be the remnants of another galaxy cannibalized by
M31. They say that an alternative possibility is that dust might
dim the core to create the illusion of a pair of separate star
clusters.

"The Hubble images intensify the mystery of what's happening in
the center of this galaxy," says Lauer. "Neither interpretation
offers a complete explanation of the M31 nuclear structure."

The double nucleus discovery is based on image analysis conducted
by Lauer, Dr. Sandra M. Faber of the University of California,
Santa Cruz, and other members of the HST Wide Field/Planetary
Camera Imaging Team.

The HST pictures show two bright spots at the heart of the M31
galaxy. The dimmer of the two "light-peaks" appears to mark the
exact center of the galaxy. The brighter peak is at least five
light years away from the true center, but corresponds to what
astronomers had previously thought was the nucleus of M31, based
on ground-based observations.

Well-known as the Andromeda Galaxy, M31 (the 31st object in a
catalog of non-stellar objects compiled by French astronomer
Charles Messier in 1774) is located only 2.3 million light years
away, making it the nearest major galaxy to our own Milky Way.
M31 dominates the small group of galaxies (of which our own Milky
Way is a member), and can be seen with the naked eye as a
spindle-shaped "cloud" the width of the full moon.

Like the Milky Way, M31 is a giant spiral-shaped disk of stars,
with a bulbous central hub of older stars. M31 has long been
known to have a bright and extremely dense grouping of a few
million stars clustered at the very center of its spherical hub.
As seen from large ground-based telescopes, the starlight blends
to resemble a single, bright, almost point-like source. Previous
ground-based observations gave little hint of the true structure
of the core, which is now revealed by Hubble.

PINPOINTING THE M31 NUCLEUS

In the 1960's the first high resolution photographs of M31's core
were obtained by Stratoscope II, a balloon-borne observatory.
The images were not as sensitive as Hubble's, and so only showed
a single bright cluster of stars.

An important clue came with observations obtained in 1986 by the
late Jean-Luc Nieto, then at the Pic du Midi Observatory in
France. He found that the bright nucleus was offset by several
light-years from the exact center of the galaxy's central bulge.
The new HST images show that the dimmer peak instead is the true
nucleus, and that the bright point of light evident
from ground-based telescopes corresponds to the brighter of two
peaks.

A CANNIBALIZED GALAXY ?

One possible explanation for the second cluster being offset from
the exact center is that it is the remnant of a smaller galaxy
that fell into M31 perhaps a billion or so years ago. The smaller
galaxy's core is the only surviving fossil relic of the galactic
collision.

A problem with the collision scenario is that the remnant core
should be torn apart by the massive black hole hypothesized to
dwell at the exact center of M31. The suspected black hole would
be located in the middle of the dimmer peak uncovered by HST.

In 1988, the first evidence for a black hole at the exact center
of M31 came from ground-based observations by Dr. John
Kormendy (now at the University of Hawaii), and independently by
Dr. Alan Dressler(Observatories of the Carnegie Institution of
Washington), and Dr. Douglas O.Richstone (University of
Michigan). Their data indicated an abrupt increase in the orbital
velocities of stars in the center of the M31 nucleus.

This led the astronomers to conclude that M31 must have a strong
but unseen concentration of mass at its center. A black hole at
least ten million times the mass of the Sun is the most likely
type of object matching these characteristics. A black hole is a
theoretical object that is so dense that even light cannot escape
its intense gravitational pull.

If such a black hole really exists, than the remnant core from
the cannibalized galaxy would be torn apart in just a few
hundred thousand years. "This is very short in cosmic time," says
Lauer. "We would have to be looking at the galaxy at a very
special time to see it now."

One way for the remnant to survive for a much longer time is if
it has its own massive black hole. Gravity from a black hole in
the remnant would hold it together against destruction from the
other black hole at the M31 center.

"In retrospect, there may be evidence for this possibility in the
spectra obtained by Kormendy, Dressler, and Richstone, says Lauer.
"One problem with this picture, however, is if the black hole in
the remnant were too big, it would distort even the true nucleus
of M31."

A DUST LANE BISECTING THE NUCLEUS?

Another interpretation of the "twin peaks" is that the bright
spot is just the outer portion of a large nuclear star cluster,
and that the central portions have been obscured by dust. A thick
ring of dust might even cut across the nucleus, creating the
illusion of two separate objects rather than one elongated
structure.

Lauer explains that the problem with this idea is that normal
galactic dust would scatter the light such that it would appear
reddened. "But this is not the case, there are no color effects
at all," he emphasizes. "This means that the dust grains would
have to be much larger than average." In our own galaxy,
however, the interstellar dust grains are roughly the same size.
"We can only guess that earlier nuclear activity in M31 would
have destroyed all the fine dust grains that would cause color
effects," says Lauer.

M31 is an ideal target for Hubble once the telescopes's optics
are improved during a space shuttle servicing mission in
December. Spectrographs aboard Hubble will dissect the light from
the two peaks of the double nucleus and determine if they are
truly separate clusters. Astronomers will be able to measure the
velocity of stars to pin down whether there is a black hole in
either or both.

This is the latest in a series of Hubble observations that have
uncovered unusual structures in the cores of galaxies. Some of
them might be fossil evidence of galactic collisions. Hubble has
also resolved very dense concentrations of stars in the cores of
other galaxies that are circumstantial evidence that massive
black holes are common among galaxies.